Prevent showing system context menu on right click on Form title bar

If you specifically want to disable showing system context menu on right click on window's title bar, you can handle WM_CONTEXTMENU:

const int WM_CONTEXTMENU = 0x007B;
protected override void WndProc(ref Message m)
{
    if (m.Msg == WM_CONTEXTMENU)
        m.Result = IntPtr.Zero;
    else
        base.WndProc(ref m);
}

If you also want to prevent the possibility of clicking on form's icon to show the context menu, then you can set ShowIcon property of the form to false:

this.ShowIcon = false;